This invention relates to a mirror having both an anti-fog property and a dimming property.
It is effective for improving visibility of a mirror for an automobile to improve anti-fog and dimming properties of the mirror. The anti-fog property is a property of removing a waterdrop which is deposited on the surface of a mirror. There is a mirror for an automobile imparted with such anti-fog property by applying a water-repellent treatment on the surface of the mirror thereby to facilitate removal of waterdrop. There is a mirror for an automobile imparted with such anti-fog property by the opposite way that is applying a hydrophilic treatment on the surface of the mirror thereby to spread waterdrops on the mirror in the form of a thin film.
The dimming property is a property of mitigating the glare of the sun light during the day. As a mirror for an automobile having such dimming property, there is a colored mirror (so-called blue mirror) which has a peak of reflection in the blue range (i.e., short wavelength range) and therefore has a mirror surface exhibiting a bluish color. The human spectral luminous efficacy has a peak at a wavelength of about 555 nm in a bright environment at which yellowish green color becomes clear. As the environment becomes darker, the peak of spectral luminous efficacy shifts in the direction of blue and the peak wavelength is shifted to about 505 nm. The blue mirror has a peak of reflectance within a range from 400 nm to 510 nm. Therefore, there is discrepancy between a reflected image from the blue mirror and the peak of the human spectral luminous efficacy during the day and the reflected image looks somewhat dark and the glare of the sun light thereby is mitigated. The reflected image agrees with the human spectral luminous efficacy during the night so that the reflected image looks relatively bright during the night thereby affording good visibility.
There is a prior art mirror for an automobile with improved anti-fog and dimming properties which is proposed in the applicant""s copending Japanese Patent Application No. 234981/1998. The structure of this prior art mirror will be-described with reference to FIG. 2. A TiO2 film 4 having a high refractive index is provided on the rear surface of a transparent glass substrate 2. On the rear surface of the TiO2 film 4 is provided a SiO2 film 6 having a low refractive index. Further, on the rear surface of the SiO2 film 6 is provided a Cr reflecting film 8 as a metal reflecting film. The thickness of the TiO2 film having a high refractive index and the thickness of the SiO2 film 6 having a low refractive index are so set that the mirror will exhibit a blue color owing to interference of light. On the front surface of the transparent glass substrate 2 is provided a TiO2 film 10 having a photocatalytic function. Further on the front surface of the TiO2 film 10 is provided a porous SiO2 film 12 having a hydrophilic property.
Light incident from a mirror surface 14 exhibits blue reflected light owing to interference between the TiO2 film having a high refractive index, the SiO2 film having a low refractive index and the Cr reflecting film 8 which constitutes the reflecting film whereby the dimming property is afforded. Owing to the action of the porous SiO2 film, waterdrops deposited on the mirror surface 14 spreads on the mirror in the form of a thin film with a contact angle of 20 degree or below whereby the anti-fog property caused by the hydrophilic property is afforded. Since the mirror for an automobile is exposed to the outside air, contaminants are deposited on the porous SiO2 film 12 which is provided on the mirror surface and the contaminants reduce the hydrophilic property of the SiO2 film. The contaminants deposited on the porous SiO2 film 12, however, are decomposed and removed owing to the photocatalytic action of the Ti02 film 10 having the photocatalytic function and the hydrophilic function of the SiO2 film thereby can be maintained.
In the prior art mirror for an automobile shown in FIG. 2, the Cr reflecting film 8 is provided as a metal reflecting film on the rear surface of the substrate 2. Further, on the front surface of the substrate 2 is provided the TiO2 film 10 having the photocatalytic function. Ti02 inherently has a high refractive index and therefore tends to cause reflection on the surface. This surface reflection and reflection from the Cr reflecting film 9 tend to produce double images in a reflected image. This is particularly so in a mirror of a small radius of curvature and a mirror of a complex radius of curvature.
Moreover, the mirror of FIG. 2 in which the two layers of the TiO2 film and the SiO2 film are respectively provided on both the front and rear surfaces of the transparent glass substrate has a complex structure with resulting high cost of manufacturing.
It is, therefore, an object of the invention to provide a colored anti-fog mirror exhibiting a blue color on the mirror surface which is capable of preventing occurrence of double images with a simple structure and a low manufacturing cost.
For achieving the object of the invention, there is provided a colored anti-fog mirror comprising a substrate, a metal reflecting film formed on a front surface of the substrate, and a hydrophilic function layer having a hydrophilic function and comprising photocatalytic substance provided on the metal reflecting film, the thickness of the hydrophilic function layer being set in such a manner that wavelength of reflected light has a peak of reflectance within a range from 400 nm to 510 nm and a mirror surface thereby exhibits a bluish color.
According to the invention, owing to the hydrophilic property of the hydrophilic function layer comprising photocatalytic substance, waterdrops deposited on the mirror surface are spread to a thin film and the anti-fog property thereby can be obtained. Further, by setting the thickness of the hydrophilic function layer comprising photocatalytic substance in such a manner that wavelength of reflected light has a peak of reflectance within a range from 400 nm to 510 nm and the mirror surface thereby exhibits a bluish color by interference between reflected light from the metal reflecting film and reflected light from the surface of the hydrophilic function layer comprising photocatalytic substance, the glare of the sun light during the day can be mitigated and excellent visibility can be obtained during the night. Further, when contaminants are deposited on the mirror surface with the result that the hydrophilic function is reduced, the contaminants are decomposed owing to the photocatalytic function of the hydrophilic function layer comprising the photocatalytic substance whereby the hydrophilic function can be restored. Further, since the distance between the metal reflecting layer and the mirror surface which causes double images is so short that occurrence of double images which can be recognized with human eyes can be prevented. Again, by causing the layer for producing interference of light to function also as the layer performing hydrophilic and photocatalytic functions, the process of manufacture can be simplified and the manufacturing cost can be reduced. In the prior art structure in which the reflecting film is provided on the rear surface of the substrate, provision of a hydrophilic function layer on the front surface of the substrate and adjustment of thickness of the layer do not contribute significantly to spectral reflectance. By only providing a hydrophilic function layer on the front surface of a substrate in the structure in which the reflecting film is provided on the front surface of the substrate and adjusting the thickness of the layer as in the present invention, color can be imparted to reflected light from the reflecting film.
The hydrophilic function layer may be made of a laminate of a photocatalytic layer comprising a photocatalytic substance and a hydrophilic layer comprising a hydrophilic substance provided on the photocatalytic layer.
According to this arrangement, the anti-fog property can be obtained by the provision of the hydrophilic layer comprising the hydrophilic substance and waterdrops deposited on the mirror surface are spread to a thin film and excellent visibility can thereby be obtained. When contaminants are deposited on the mirror surface with resulting reduction of the hydrophilic function, the contaminants are decomposed owing to the photocatalytic function of the photocatalytic layer comprising the photocatalytic substance whereby the hydrophilic function is restored.
A reflectance adjusting layer which is made of substance having a lower refractive index than the photocatalytic layer may be provided between the metal reflecting film and the photocatalytic layer.
According to this arrangement, by setting the thickness of the reflectance adjusting layer at a proper value, reduction in reflectance caused by producing reflected light of bluish color which is of a low spectral luminous efficacy can be compensated and excessive reduction in reflectance of the mirror thereby can be prevented.
The thickness of the hydrophilic layer may be selected within a range from 10 nm to 50 nm. By selecting the thickness within this range, performance of the photocatalytic function of the photocatalytic layer comprising the photocatalytic substance on the surface of the hydrophilic layer comprising the hydrophilic substance can be facilitated.
The hydrophilic function layer may be made of a layer comprising photocatalytic substance having a hydrophilic property.
A reflectance adjusting layer which is made of substance having a lower refractive index than the layer comprising photocatalytic substance having a hydrophilic property may be provided between the metal reflecting film and the layer comprising photocatalytic substance having a hydrophilic property.
The hydrophilic function layer may be made of a layer comprising a mixture of photocatalytic substance and hydrophilic substance.
A reflectance adjusting layer which is made of substance having a lower refractive index than the layer comprising a mixture of photocatalytic substance and hydrophilic substance may be provided between the metal reflecting film and the layer comprising a mixture of photocatalytic substance and hydrophilic substance.
The hydrophilic substance may for example be made of a hydrophilic inorganic oxide such as SiO2. Since SiO2 inherently has a hydrophilic property, the hydrophilic function can be obtained by employing SiO2. The structure comprising the hydrophilic layer comprising the hydrophilic substance on the outermost surface has excellent resistance to scratches which protects the photocatalytic layer comprising the photocatalytic substance.
In a case where the metal reflecting film is made of Cr, the reflectance adjusting layer may be made of Al2O3, ZrO2 or SnO2. When Cr is employed for the metal reflecting film and the peak of reflectance is provided within the range from 400 nm to 510 nm, this arrangement is effective for securing proper reflectance (40%) as a mirror for an automobile. More specifically, by inserting the reflectance adjusting layer to enlarge difference in refractive index between the adjacent layers, reflectance can be increased in the entire visible light region and excessive reduction in reflectance can thereby be prevented.
The photocatalytic substance may be made of, for example, TiO2. Since TiO2 has a high photocatalytic function, an excellent contaminant decomposing function can be obtained. Since, further, TiO2 has the hydrophilic property in itself, the anti-fog property can also be obtained, if it is arranged at a proper location.
The surface of the hydrophilic function layer may be made porous. By this arrangement, a hydrophilic function of a higher degree can be provided.
Embodiments of the invention will be described below with reference to the accompanying drawings.